Interventional medicine is seeing a growing trend toward minimally invasive and catheter-based therapy, especially in cardiovascular and intracardiac procedures. Minimally invasive therapy can greatly reduce patient trauma, recovery time, overall costs, and ultimately patient mortality and morbidity of procedures. Catheter-based interventions, as a form of minimally invasive surgery, can decrease hospitalization time and greatly lower patient morbidity compared to traditional methods. In cardiology, interventions are often performed using catheters to navigate through blood vessels to the heart starting from a small incision in the leg. The physician manipulates the catheter from outside the body by using pull cables along the flexible catheter. Percutaneous catheter procedures are hindered by a lack of precise tip manipulation when actuation forces are transmitted over the length of the catheter. These manual catheters lack the precision and dexterity needed to reach the desired locations inside the heart efficiently and effectively. Because pull cable forces must be translated along the length of the catheter, the translation of forces becomes overshadowed by friction and stiction along the catheter between the physician's hand and the catheter tip. Therefore, current catheters cannot match the manipulative capability or proprioceptive feedback that is available during more invasive open chest procedures.
In cardiology, accuracy in the range of a few mm is needed, where heart motion and flowing blood tend to perturb the position of the catheter tip. In addition, the catheter must be on the order of 3-4 mm in diameter to be able to navigate the blood vessels.
Development of a robotic or “active” catheter that can provide local, precise actuation relative to the tip of the catheter has the potential to overcome some of these challenges, but additional sensory information is necessary for precise control and force feedback. Shape memory alloy (SMA) actuators have a large force to volume ratio, thus making them an ideal candidate for minimally invasive tools. Unfortunately, the actuators also exhibit a large hysteresis, which makes them difficult to control, thus model-based open-loop control as well as feedback control using the SMA actuator's own electrical resistance have not led to reliable and controllable actuators, and were unable to provide measurement over the desired long-range of motion.
Current catheters do not have position sensing. In some cases the physician uses biplane fluoroscopy and manual manipulation to maneuver the catheter. Electronically actuated systems could improve the precision of manipulation, but feedback is still needed for good control. The need for position sensing is crucial to obtaining robust control of an active catheter. There are many position sensing techniques, but few are easily integrated into a catheter. Linear Variable Differential Transducers (LVDTs), capacitive sensors, and similar devices cannot be scaled down to a 2-3 mm diameter tubular structure required for intracardiac catheter applications.
The addition of separate position sensors can enable precise control of shape memory alloy (SMA) actuators. Active catheters with local SMA actuation can potentially provide the desired manipulation of a catheter tip, but to date a method does not exist to easily integrate small-volume compliant sensors to provide position and/or force feedback for the actuators. A number of research groups have attempted to build active catheter devices using different materials and methods. Some of these groups have used shape memory alloy actuators, but only one device has incorporated closed-loop feedback, and yet only limited success was achieved. Other devices rely on open-loop control of SMA actuators, which is challenging due to their large hysteresis.
Robotic or computer-aided surgical systems do not currently provide force information to the user. Although they have developed a master-slave user-interface for operation of their system, there is currently no ability to sense or feedback position of the catheter. Another group has developed a system for magnetic guidance of a catheter tip, but this system relies on large, bulky magnets to be placed within the interventional suite.
Currently it is very difficult to obtain force information from a catheter due to difficulty in integrating a small enough sensor on the tip of the catheter. Standard interventional catheters do not provide the physician with appreciable force feedback due to the inability of the catheter structure to effectively transmit forces from the tip down the length of a long floppy catheter to the physician. Current force sensors either cannot sense the small forces imparted on a catheter or do not fit the form factor or scale required for a sensor on a catheter. Even current robotic catheter systems cannot employ effective force sensors because there is no appropriate device that would fit on the end of a catheter.
The inventors have previously developed SMA actuators that were laser-machined from nitinol tubing to achieve up to 2N of force at 20% elongation, and have shown that if good position sensing is provided, the actuators can be controlled precisely regardless of their material properties. It is believed that these actuators are capable of exerting 500 mN forces over 500 μm, or up to 33% elongation. Therefore, development of a useful position sensor that can accurately detect this large change in elongation while providing little mechanical resistance becomes a challenge. External strain gauges are known to be difficult to mount on the actuator, and other sensor modalities, such as Hall Effect sensors, can be too costly, large, or complicated to implement. In addition, integration of separate technologies into this laser-machining platform is known to be difficult.
Because SMA actuators are highly nonlinear with large hysteresis, a method and device for obtaining feedback to control the actuators and achieve precise positioning is needed. Although there have been numerous attempts to use the resistance of the actuators themselves as position feedback, the nonlinear and hysteretic dependence of the resistance on both stress and temperature make it an elusive feedback variable. What is needed is a sensor that could be incorporated with an actuator, such as a SMA actuator or wire pulley actuator, and sense the actuators actual displacement directly to enable an accurately steerable mechanism. Local actuation of the catheter tip relative to a nearby reference point would provide the physician a higher degree of control. Furthermore, local position and force sensing would allow for closed-loop control of such an active catheter and better response to environmental perturbations. What is further needed is a system that incorporates position and force sensing along the length of the catheter, with the ability to actuate the catheter mechanically, and can provide sufficient force and dexterity as well as more accurate control of the catheter tip in an open environment like the heart chambers.